Wuerhosaurus, one of the last of the stegosaurs, lived about 20 million years after the time, in the late Jurassic, when these plant-eaters were in their prime. It is not clear why the stegosaurs died out, but most scientists believe that their demise was related to the rise of the other main group of armored dinosaurs-the ankylosaurs. This assumes that stegosaurs and ankylosaurs shared the same niche in the environment, and that the emerging ankylosaurs developed some kind of advantage over the more primitive stegosaurs. Just what that advantage could have been, however, we can only guess at.

Wuerhosaurus was one of the most unusual members of a very unusual group. Whereas the plates on the backs of other stegosaurs were tall and triangular, those on Wuerhosaurus were long, low, and rectangular. And unlike the staggered arrangement of Stegosaurus's plates-but like those of other Chinese stegosaurs-those on Wuerhosaurus were probably arranged in matching pairs.

While most researchers agree that Wuerhosaurus, like its close relatives, probably had four tail spikes, there is as yet no firm evidence for them. We do know that the hips, like those of other stegosaurs, were fused into a single, broad, platelike structure across the rump, and that the vertebrae at the base of the tail had tall neural spines that may have helped Wuerhosaurus to suspend or maneuver it tail.

Lack of evidence continues to frustrate our attempts to fully flesh out this dinosaur. Up until now, only fragmentary skeletons and a few isolated bits and pieces have been found. There are, therefore, large chunks of the animal that we can only speculate about, and our total picture remains incomplete. We may soon learn more from a new species of Wuerhosaurus that was found in the late 1980s by joint excavations between Chinese and Canadian paleontologists.